fix: NameError on logger in custom endpoint model discovery

get_available_models() references 'logger' in the except block of
the custom endpoint fetch (added in PR #18), but 'logger' is never
imported or defined in api/config.py. When the custom endpoint is
unreachable (the normal case -- most users don't have a local LLM),
the except handler raises NameError: name 'logger' is not defined,
which propagates as a 500 on every GET /api/models request.

This broke 7 test_sprint11 tests and caused the model dropdown to
fail for all users regardless of whether they have a custom endpoint.

Fix: replace logger.debug() with a silent pass -- the exception is
expected when no local LLM is configured and needs no logging.

Tests: 237 passed, 0 failed.

api/config.py

@@ -510,8 +510,8 @@ def get_available_models() -> dict:
                 if model_id and model_name:
                     auto_detected_models.append({'id': model_id, 'label': model_name})
                     detected_providers.add(provider.lower())
-        except Exception as e:
+        except Exception:
-            logger.debug(f"Failed to fetch models from custom endpoint: {e}")
+            pass  # custom endpoint unreachable or misconfigured -- fail silently
 
     # 5. Build model groups
     if detected_providers: